1. Field of the Invention
This invention relates to communication over noisy lines and more particularly, but not exclusively, to communication and telecontrol over the electricity mains network.
2. Description of Prior Art
A line network communications system commonly encounters interference problems arising from a variety of sources. Severe interference may result in data loss during transmission, necessitating repeat commands or ultimately, periods of communication breakdown. Occurrences of this nature may hinder the operation of an effective communications system.
In a communication system data may be codified and the codified data signal sent on to a carrier or baseband signal to be transmitted. This forms a spread spectrum signal in which the effective bandwidth of the data is considerably increased, thereby reducing sensitivity to interference because of the enhanced signal to noise gain.
A receiver including a matched filter may recover the spread spectrum signal. Recording of the recovered signal may only occur if the receiver internal reference code matches the code within the codified data signal. In addition, the phase of the internal code must be searched for, tracked and locked on to in order to maintain synchronism with the data signal code allowing receival to occur. It will be apparent that the effectiveness of this phase synchronisation, the searching therefor, tracking and locking thereto, in the presence of interference within the received spread spectrum signal reflects the effectiveness of the communications system, in particular under adverse transmission conditions.